This patchset adds support for keeping mount ownership information in
the kernel, and allow unprivileged mount(2) and umount(2) in certain
cases.
This can be useful for the following reasons:
- mount(8) can store ownership ("user=XY" option) in the kernel
instead, or in addition to storing it in /etc/mtab. For example if
private namespaces are used with mount propagations /etc/mtab
becomes unworkable, but using /proc/mounts works fine
- fuse won't need a special suid-root mount/umount utility. Plain
umount(8) can easily be made to work with unprivileged fuse mounts
- users can use bind mounts without having to pre-configure them in
/etc/fstab
The following security measures are taken for unprivileged mounts:
- only allow submounting under mounts which have a special mount flag set
- only allow mounting on files/directories writable by the user
- limit the number of user mounts
- force "nosuid,nodev" mount options
Changes from the previous submissions:
- add mount flags to set/clear mnt_flags individually
- add "usermnt" mount flag. If it is set, then allow unprivileged
submounts under this mount
- make max number of user mounts default to 1024, since now the
usermnt flag will prevent user mounts by default
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